This invention relates to a turntable platter for record players, consisting of a glass and a plastic layer, one on top of the other, which confers to the platter the inherent characteristics of these materials, namely, flatness, rigidity, attractive appearance and light-weight, but above all they permit considerably improved quality of sound reproduction.
In order to understand the importance of the invention it should be above all emphasized that during the last few years in the record player industry, considerable evolution has taken place in the electronic components, while the "mechanical" progress has been much slower.
It is only necessary to go back to 1978 to find turntable platters for high quality record players first being widely provided with a "mat". This accessory was able to ensure greater fidelity in sound reproduction as it acted as a damping means for the primary spurious vibrations (set up by the tracking of the stylus over the groove surfaces on the gramophone record) and of secondary spurious vibrations from the plinth, with both sets of vibrations being amplified by the resonance of the metal platter. The introduction of this feature appears to have provided an appreciable but--as will be seen later on--insufficient improvement in the sound reproduction quality.
The most critical point of the sound reproduction chain "armgramophone record-platter-plinth" of a record player is without doubt the platter, which is still today being made of metal alloy, owing to the high resonance of sound which is a distintive property of metals.
The rotation of a gramophone record on the turntable platter is always accompanied by vibrations produced by the stylus tracking the side surfaces of the groove. These vibrations are a few microns in amplitude and generate spurious vibrations throughout the entire pickup system: gramophone record, arm, platter, plinth.
Such vibrations can be detected by listening to their propagation across the platter and plinth with the aid of, for example, a doctor's stethoscope placed under the plinth.
the generation of spurious vibrations is therefore inevitable. They are transmitted in the air at a velocity of 343 m/s, while the velocity of propagation in metals is about 6000 m/s.
Turntable platters for record players built up to now, as stated earlier on, of metal alloy, constitute a true centre of resonance for the vibrations.
In order to attenuate disturbances generated by such vibrations in HIFI equipment, use has been made--as already mentioned since 1978--of mats placed between the gramophone record and the platter. These mats are made of various materials such as: chamois leather, pigskin, rubber, caoutchouc, plastic, silicone and so on.
Yet another type of cover is used consisting of a thin-walled container filled with oil.
However, such mats are not free from defects which could actually give rise to a loss of sound information.
Such loss can exceed 2 dB at certain frequencies with a certain amplitude, therefore it is prefectly noticeable and can appreciably change the timbre of certain instruments. In all cases, the mat can only absorb part of the spurious vibrations.
Furthermore, when the mat is thicker than 3 or 4 mm (some are up to 7 mm thick), it is necessary to adjust the pickup arm height in order to restore the horizontal position of the pickup head, and prevent increased distortion. This operation is not easy to carry out in a fault-free manner.
To sum up therefore: the problems linked with turntable platters of today are:
(a) a resonance effect due to the metal alloy of which the platter is made; and PA1 (b) the insufficiency of the mat to absorb parasitic vibrations without detracting from the quality of the sound reproduction.